1. Field of the Invention
The present invention generally relates to an ink-jet printer and more in particular to an apparatus for detecting deflection of a charged ink droplet for use in an ink-jet printer of the deflection control type.
2. Description of the Prior Art
The conventional deflection control type ink-jet printer is schematically shown in FIG. 1. As shown, the ink-jet printer comprises an ink cartridge 1 from which ink is supplied through a pump 2 to an accumulator 3. Then, the ink under pressure is supplied from the accumulator 3 to an ink-jet head 5 through a filter 4. The head 5 is provided with an electrostrictive vibrator 5a which imparts vibration of a constant frequency to the ink under pressure when excited. Thus the vibration-imparted ink under pressure is discharged out of the nozzle of the head 5 in the form of a jet. This ink jet is then regularly separated into ink droplets at a predetermined position in the downstream of the nozzle, whereby the separation frequency coincides with the frequency of the vibration.
A charging electrode 6 is disposed at the position where the ink jet is separated into ink droplets and a potential is applied to the electrode 6 for charging the separated droplets. Such a charging potential is variable in level in a stepwise fashion, and no-recording state, i.e., image signal "0", is defined to be 0 level, e.g., ground potential. Because of the fact that the charging potential must be applied in the form of pulse and that each of the stepped charging potentials must be applied in conformity with the phase of the formation of ink droplets, the phase of the charging potential pulses is set for the excitation phase of the electrostrictive vibrator 5a of the head 5 through the charging phase retrieval procedure. That is, the output from a clock pulse generator is supplied to an excitation amplifier 41 which then produces a sinusoidal wave in synchronism with the clock signal, said sinusoidal wave being applied to the electrostrictive vibrator 5a of the head.
The output signal from the clock pulse generator is also supplied to a phase control circuit 30 which then produces charging clock pulses having a constant pulse width and a predetermined phase difference with respect to the input clock to the circuit 30. Then the output from the circuit 30 is supplied to a retrieval amplifier 44 which in turn produces retrieving pulses having the same phase as that of the charging clock pulses and a constant level with the polarity either same as or opposite to that of the charging potential. These retrieving pulses are supplied to the charging electrode 6. Then the charge of an ink droplet is detected. That is, it is monitered whether or not a charge detecting circuit has generated a charge detecting signal during production of a predetermined number of ink droplets, and if such a signal has been generated, the phase retrieval procedure is terminated; on the other hand, if such a signal has not been generated, the phase is shifted by one step at the phase control circuit 30 thereby shifting the charging potential pulses over a predetermined phase.
In this manner, upon completion of setting of an appropriate phase for the charging potential pulses with respect to the output clock from the clock pulse generator, charging potential pulses for printing or recording, which are variable in level in a stepwise fashion and wider than the charging potential pulses for phase retrieval located at the center of the pulse width, are applied to the charging electrode 6, so that printing or recording operation takes place. That is, when the charging potential which may vary in level in association with the charging clock signal is applied to the charging electrode, ink droplets are charged in accordance with the level of the charging potential, and, therefore, the ink droplets are deflected through the interaction between the amount of charges of the droplets and the electric field formed between deflection plates 7.sub.1 and 7.sub.2. If the image signal is at "0" level, the charging potential is set at "0" level. So, an ink droplet is not charged in this case and therefore it will be collected by a gutter 9.
One of the problems associated with such an ink-jet printer as described above is the deflection error of an ink droplet due to changes in conditions such as temperature and pressure of the ink. Thus, heretofore, various attempts have been made to adjust the charging potential and the temperature and pressure of the ink by detecting the temperature and pressure of the ink, the flying velocity of an ink droplet, and the amount of charge or deflection.
In the system shown in FIG. 1, there are provided three charge detecting electrodes 48a-48c which are connected to charge detectors 48a'-48c', respectively. It is so arranged that the output signals from the charge detector 48a' and 48c' indicate insufficient deflection and excessive deflection, respectively, and the output signal from the charge detector 48b' indicates proper deflection, whereby ink pressure and/or charging potential are adjusted such that the charge detector 48b' supplies a charge detecting signal. It may be so structured that only one or two charge detecting electrodes are provided. It has also been proposed to change ink pressure and/or charging potential in a manner of geometric series in order to carry out such adjustments quickly. It has also been proposed to use non-contact type detecting electrodes. When use is made of such impingement type or non-contact type deflection detecting electrodes, accuracy in arrangement is critical because the print dot distribution and dot shift are influenced by the arrangement of the electrodes. Moreover, when an ink droplet impinges upon the detecting electrode, ink splashes are scattered around thereby making the detecting electrode leaky and thus allowing to carry out only inaccurate detection. In the prior art, the detecting electrodes are also prone to produce noises due to external noise source or ON/OFF operation of the charging potential.